Transcript Chapter-3 Clastic transport and fluid flow

CLASTIC TRANSPORT AND
FLUID FLOW
CHAPTER 3
Chapter-3 Clastic transport
and fluid flow
 Weathered
rock and minerals fragments
are transported from source areas to
depositional sites (where they are subject
to additional transport and redeposition) by
three kinds of processes:
1- dry (non-fluid assisted), gravity-driven
mass wasting processes such as rock
fall and rock slides;
 2-
wet (fluid assisted), gravity-driven mass
wasting processes (sediment gravity
flows) such as grain flows, mudflows,
debris flows, and some slumps; and
 3-
processes that involve direct fluid flows
of air, water, and ice.
 Mass
wasting
 Fluid flow
 Reynolds Number
 Froud Number
 Entrainment, transport and
deposition of clasts
 Transport
Mass Wasting
 Mass
wasting processes are important
mechanisms of sediemnt transport.
 Although they move the soil and rock
debris only short distances downslope ,
these processes play a crucial role in
sediment transport by getting the products
of weathering into the longer-distance
sediment transport system.
Mass Wasting
 In
dry mass-wasting processes, fluid plays
either a minor role or no role at all.
 In rock or talus falls, clasts of any size
simply fall freely.
 Downslope movement of bodies of rocks
or sediment in slumps or slides glide
downslope en masse without significant
internal folding or faulting. Fluids near the
base provides lubrication and promotes
failure along slippage surface.
Reynolds Number
 Re=
2rVp/
 Sir Osborne Reynolds addressed the problem
of how laminar flow changes to turbulent flow.
 The transition from laminar to turbulent flow
occurs as velocity increases, viscosity
decrease, the roughness of the flow boundary
increases, and/or the flow becomes less
narrowly confined.
Froud Number

The Froud Number is the ratio between fluid
inertial forces and fluid gravitational forces.
 It compares the tendency of a moving fluid (and
a particle borne by that fluid) to continue moving
with the gravitational forces that act to stop that
motion.
 The force of inertia express the distance traveled
by a discrete portion of the fluid before it comes
to rest.
 Like reynolds Numbers, Froud numbers are
dimensionless.
Froud Number
• Fr =
fluid inertial forces
.
•
gravitational forces in flow
Deposition: What forces control
the settling of particles?
 As
soon as a particle is lifted above the
surface of a bed, it begins to sink back
again.
 The distance that it travels depend on the
drag force of the current, and the settling
velocity of the Particle.
 The velocity at which a clast settles
througha fluid is calculated using
STOKES’ LAW of settling
Stokes’ Law of settling
 The
gravitational force pulling the particle
down versus the drag force of the fluid
resisting this sinking.
 The particle will be initially accelerate due
to gravity, but soon the gravitational and
drag forces reach equilibrium, resulting in
a constant “Terminal Fall Velocity”.
 The
drag force exerted by a fluid on a
falling grain is proportional to the fluid
density (F), the diameter (d) of the grain
(in centimeters), the drag coefficient (CD)
and the fall velocity (V).
 Drag
force= CD π (d2/4) (F V2/2)
Drag force
 Upward
force due to buoyancy of the fluid
is: Fupward = 4/3 π (d/2)3 Fg
 Downward forces due to gravity:

Fg = 4/3 π (d/2)3  sg, where ps is the density
of the particle.
 Drag
force= Fg - Fupward
Drag force= Fg - Fupward









CD π (d2/4) (pF V2/2)= 4/3 π (d/2)3 sg - 4/3 π (d/2)3 Fg
V2= 4gd (s- F)
3 C D F
For low laminar flow at low concentrations of particle and low
Reynolds numbers, CD is equal to 24/Re.
V = 1/18 ([ s-  F] g d2 /u) - Stokes’ Law of settling
V-velocity, g-gravity, u-viscocity, even the differnce in density (s-  F)
is constant for a given situation. It can be substitute for C
C = ( s-  F)g
18u
Stokes law reduces to V=CD2
Stokes law reduces to V=CD2
 When
density and viscosity are constant,
settling velocity increases with the
diameter of the particle.
 Larger grains fall faster.
 Settling
velocity decreases with higher
viscosities and increases with denser
particles. C = (s- F)g

18u
Implications of the Stokes
Law
 High
density minerals settle more rapidly
than low density minerals.
 Slow-moving, highly viscous fluids such as
mudflows and density currents can
transport coarser-grained materials than
less viscuos fluids such as rivers and the
wind, despite the normally higher velocity
of these less viscous fluids.
 Lower
temperatures will increas viscosity
decreasing the fall velocity.
 Because of turbulence, coarser particles
fall more slowly than predicted.
 Non-spherical flakes such as mica will
settle more slowly than spheres with the
same density.
 Angular grains will generate small
turbulent eddies that retard settling
velocity.
Hydraulic equivalency
 The
term refers to clasts that settle at
identical velocities despite substantial
differences in size, shape, angularity, and
density.

ie. Sediment mixes of fine grained, silt-size
magnetite, fine sand-size biotite flakes, and
medium sand-size quartz.